This is a pet peeve of mine. You're likely taking a greater risk than your "boss" here is. You should be asking and getting at least 30% if you're going into an unfunded, unproven, un-DEVELOPED company. I start negotiations at 50% in these situations and make a potential partner defend a reduction from that point.
But the top two comments in this thread nailed it. This is not going to end well for you in all likelihood. I'd pass on it.